In machine construction, and particularly in the construction of coordinate-measurement machines, an elongate movable machine part, such as the quill which carries the measurement tool, extends into the working or measurement region, and it is frequently necessary to protect the tool from unintended collisions. And it is further necessary to provide assurance against any possibility of injury to the operating personnel by moving parts of the machine.
For this purpose, it is known, inter alia from EP-Al-116,807 to produce an envelope-shaped light curtain around the quill of a measurement machine of bridge-type construction; when the curtain is pierced, the machine is stopped. This known protective device requires a ring of reflectors or receivers at the lower end of the quill which carries the measurement sensor. But such a device has disadvantages. On the one hand, the reflector ring and the sensor-head parts located below the reflector ring are unprotected and can be damaged in the event of a collision. On the other hand, the physical size of the ring of reflectors effectively increases the transverse dimensions beyond the amount needed to accommodate the quill. Further, the ring of receivers is an obstacle to the mounting or dismounting of the quill and must first be removed.
A coordinate-measurement machine of portal-type construction is also known to provide quill protection via tension springs; these springs are distributed around the quill and are at different electrical potential than the body of the machine.
In the event of a collision, the machine is stopped upon contact between quill and tension springs. In this case, a ring is also necessary at the lower end of the quill, in order to tension the springs. Furthermore, this known apparatus affords protection only when the obstacle or the quill itself consists of electrically conductive material. But measurement machines, including the quill, are increasingly being made of granite.
It is further known to use ultrasonic sensors for collision protection, among other things in the case of rail vehicles. In such case, a sonic transmitter sends out a pulse in the direction of movement of the vehicle, and, from its travel time or the travel time of its echo, the distance from an obstacle is determined; a switching process is initiated should said time be less than a predetermined safety distance. However, this principle does not readily lend itself to providing anti-collision assurance for transverse movement of the quill of a measurement machine; the sonic sensors have a certain lag time and as a result do not permit measurement within short-distance ranges of less than about 250-mm. In the case of measurement machines, however, in most cases the object to be measured itself forms the obstacle, and it must be possible to bring measurement probes as close as possible to the object to be measured. Furthermore, to assure protection against collision for an elongate part, it is necessary to use a plurality of sonic sensors, the echo signals of which, however, interfere with each other.